Photolithography is a technique for transferring images onto semiconductor or other substrates. There are two fundamental types of photolithography systems. A first type, referred to as image-projection lithography, uses master patterns, referred to as masks or reticles, and a projection system for projecting the image on the mask on a substrate. A second type of system, referred to as a maskless or direct-write system, forms images directly onto the substrate by scanning (or “writing”) beams of light on the substrate. Maskless systems are used to generate the masks for the image-projection lithography. However, systems with masks are generally faster and better suited for high-volume commercial applications than the direct-write systems.
In image-projection lithography, an image formed on a mask is transferred to a substrate by a projection device. In one type of image-projection system, known as a wafer stepper, the entire mask pattern is projected onto the substrate at once, patterning one portion of the substrate. The mask is then moved (“stepped”) relative to the substrate, and another portion of the substrate is patterned. In another type of lithographic projection apparatus, known as a step-and-scan apparatus, portions of the substrate are irradiated by progressively scanning the mask with a projection beam while synchronously scanning the substrate parallel to this direction. In both types of systems, an image is projected onto a photosensitive layer, referred to as a resist, layered on the surface of the substrate. Each mask includes a pattern corresponding to a layer of circuit components or interconnects to be formed on the substrate. A number of patterns are exposed and processed to build up the three-dimensional structure of an integrated circuit.
After exposure, the resist is developed leaving only a selected pattern of resist on the wafer corresponding to the exposed image. Since resists “resist” etching of the substrate below them, the pattern developed in the resist is transferred to the substrate by a subsequent etch step. The resist may be of a positive or negative type, which refers to the fact that the exposed resist may selectively remain on the wafer or be removed from the wafer in the development step.
Unfortunately, due to decreasing design rules and the wide use of RET (Resolution Enhancement Techniques) such as OPC (Optical Proximity Correction) and PSM (Phase Shift Masks), the masks used in image-projection systems have become increasingly difficult and expensive to make. Since many masks are needed to form the multiple patterns required to manufacture an integrated circuit, the time delay in making the masks and the expense of the masks themselves is a significant cost in the manufacture of semiconductors. This is especially so in the case of smaller volume devices, where the cost of the masks cannot be amortized over a large number of devices. Thus, it is desirable to provide a fast apparatus for making semiconductor chips while eliminating the need for expensive masks. It is also desirable to improve the obtainable resolution of optical lithography. Further, such a device may be useful for directly patterning a small number of substrates, such as runs of prototype devices, and for making masks.
A method to improve the resolution obtainable with conventional masks is described in UV thermoresists: sub-100-nm imaging without proximity effects, Gelbart, Dan, Karasyuk, Valentin A., Creo Products Inc., Proc. SPIE Vol. 3676, p. 786-793, Emerging Lithographic Technologies III, Yuli Vladimirsky; Ed. 6/1999. In this method, microlens arrays are used in combination with image-projection systems to break the image into an array of high-resolution spots that are scanned between pulses in a conventional stepper, forming a complete image. Since the spots are separated one from another, such systems eliminate optical proximity effects. With a combination of a thermal photoresist, this method provides for improved resolution with conventional masks. However, it still requires the use of a mask or reticle.
A method that eliminates the need for masks has been proposed in “A Microlens Direct-Write Concept for Lithography,” Davidson, Mark, SPIE VOL. 3048, PP. 346-355, 1997 (Spectel Company, Mountain View Calif.). In this system microlens arrays have been proposed for use in direct-write systems in combination with parallel light beams for the purpose of obtaining high resolution and higher throughput. A beam splitter produces an array of parallel beams which are individually modulated by an array of piezoelectric discs in a parallel-array Michelson interferometer modulator. The modulated beams are imaged by a microlens array onto a substrate in a multi-spot grid pattern.
Another direct-write lithography employing a microlens arrays is described in “Microlens scanner for microlithography and wide-field confocal microscopy,” U.S. Pat. No. 6,133,986 issued to Kenneth C. Johnson Oct. 17, 2000. In this system a parallel array of modulated beans is provided by a Digital Micromirror Device (DMD), described by J. B. Samsell in “An Overview of the Performance Envelope of Digital Micromirror Device (DMD) Based Projection Display Systems,” Society for Information Display 1994 International Symposium (San Jose, Jun. 12-17, 1994). Each beam is imaged through a corresponding element in a microlens array onto a substrate in a multi-spot grid pattern.
Although both the Davidson and the Johnson systems eliminate the need for masks, they do not improve the resolution obtainable from optical based lithography. Accordingly, it is desirable to provide a practical, maskless, direct-write system with improved resolution and throughput for improved mask making and wafer lithography.